Researchers from the University of Chicago have found that rates of autism and intellectual disability in the US correlate with incidence of genital malformation in newborn males at county level - an indicator of fetus exposure to harmful environmental factors, such as pesticides.

The research team, including Prof. Andrey Rzhetsky, recently published their findings in the journal PLOS Computational Biology.

Past research has linked environmental factors with the development of autism.

To investigate this association further, the researchers analyzed medical information from an insurance claims database that involved almost 100 million patients across the US.

As an indicator of parental exposure to environmental toxins, the team looked at the levels of congenital malformations of the reproductive system in males.

Researchers have found that autism rates strongly correlate with rates of genital malformations at birth in males across the US - an indicator of fetus exposure to harmful environmental factors.

They note that male fetuses are very sensitive to certain environmental toxins, and such exposure is believed to lead to reproductive malformations existing at birth, including micropenis and undescended testicles.

On assessing the incidence rates of autism and intellectual disability (ID) in their dataset county by county, the team found that every 1% increase in malformations was linked to a 283% increase in autism and a 94% increase in ID in that same county.

They also found that almost all areas with higher autism rates had higher ID rates. The researchers say this supports the presence of harmful environmental factors in these areas.

Furthermore, the researchers discovered that male children with autism were almost six times more likely to have genital malformations at birth.

Effect of environmental exposure 'surprisingly strong'

Commenting on the findings, Prof. Rzhetsky says:

"Autism appears to be strongly correlated with rate of congenital malformations of the genitals in males across the country. This gives an indicator of environmental load and the effect is surprisingly strong."

The research team also found that viral infections in males were linked to significant increases in incidence rates of autism and ID.

Since exposure to environmental toxins is associated with the weakening of human immune systems, the researchers say this finding supports the theory that environmental exposure may be linked to autism and ID incidence.

On analyzing other potential factors that may influence autism and ID incidence rates, the researchers found that state-specific law had a significant impact. For example, state-mandated diagnosis of autism by a clinician or pediatrician for consideration in the special education system was linked to a 99% decrease in autism and ID incidence rates.

Fast facts about autism

Approximately 1 in 88 children in the US has been identified with an ASD.

ASDs are almost five times more common among boys than girls.

Children born to older parents are at higher risk for ASDs.

Income appeared to have a small impact on incidence rates. Every extra $1,000 income above the county average was linked to a 3% increase in autism and ID rates, but this was considered a "weak link."

The researchers note there were some limitations to their study that could prevent the findings from being generalizable. For example, ease of access to data may differ between counties, or there could be "uneven genetic distribution."

"For future genetic studies we may have to take into account where data were collected, because it's possible that you can get two identical kids in two different counties and one would have autism and the other would not," explains Prof. Rzhetsky.

But overall, he says the team interprets the results of the study as "a strong environmental signal."

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